A government whistleblower complained to the House Ways and Means Committee on July 29 about “inappropriate efforts to influence” the mandatory Internal Revenue Service audit program which is auditing the tax returns of President Donald Trump and Vice President Mike Pence.

The chairman of the committee, Rep. Richard Neal (D-Massachusetts) said the panel received “an unsolicited communication from a Federal employee setting forth credible allegations of “evidence of possible misconduct” —specifically, potential “inappropriate efforts to influence’ the mandatory audit program.”

The Washington Post reported Thursday that “an Internal Revenue Service ­official has filed a whistleblower complaint reporting that he was told that at least one Treasury Department political appointee attempted to improperly interfere with the annual audit of the president’s or vice president’s tax returns.”

Neal wrote to Treasury Secretary Steven Mnuchin on Aug. 8, seeking documents and communications regarding specific employees at the department and the Internal Revenue Service. Mnuchin missed the deadline to begin turning over the documents.

Trump’s tax returns have been kept secret, prompting questions and subpoenas from Democrats. He is the first president since Richard Nixon to keep his taxes private (although Nixon’s returns were initially leaked).

American political journalist and author Cokie Roberts explained the history of presidents turning over taxes in a February interview with NPR.

“It’s been standard from Nixon on for presidents and presidential candidates to let the public see what they’ve paid, but not everyone has handled it the same way,” said Roberts. “Gerald Ford, Nixon’s successor, provided a summary of his taxes. Some candidates have just turned over a couple of years’ worth of documents. Others have provided returns for many years. But none has totally stonewalled the way Trump has.”

New York District Attorney Cyrus Vance subpoenaed eight years of Trump’s personal and corporate tax returns on Sept. 16 from Mazars USA, Trump’s accounting firm. Earlier, a House committee similarly subpoenaed eight years of accounting records from Mazars USA.

In both suits, Trump sued the person subpoenaing and Mazars. He lost against the committee and Mazars but appealed the verdict, effectively delaying the process.

Some democrats believe the tax returns will show proof of hush-money payments before the 2016 election. Trump’s former lawyer, Michael Cohen, paid adult-film actress Stormy Daniels $130,000 to keep quiet about her alleged affair with the president during the election. Cohen was compensated by the Trump organization. Vance’s office is investigating whether Trump’s reimbursement to Cohen violated New York state law.